In the field of this invention, it is known for television systems and the like to use an interlaced scan line technique in order to minimise the signal bandwidth used. With the interlaced scan line technique, an image is displayed as two scan line fields; one scan line field comprising the odd horizontal lines of the image; and the second scan line field comprising the even horizontal lines of the image. In this manner, odd and even scan line fields of images are alternately displayed, for example at a rate of 60 fields per second for NTSC (National Television System Committee) systems and at a rate of 50 fields per second for PAL (Phase Alternating Line) systems.
A common problem with using the interlaced scan line technique is that alternating the display of odd and even lines can result in flickering of the image. Flickering can be seen as an aliasing phenomenon, and is the effect caused when high frequency image energy aliases to low frequency image energy, thereby resulting in low frequency flicker where high spatial vertical frequencies are present. Flickering is most visible on cathode ray tube (CRT) monitors, due to luminescence fading of even horizontal lines of the image of the phosphor coating. The root cause of flickering is vertical modulation of an image caused by the fading of the phosphor coating, which leads to aliasing.
To avoid such flickering, it is known to use a vertical image filter, commonly known as a ‘flicker filter’. A flicker filter effectively attenuates the values of vertically adjacent pixels to decrease the difference between adjacent odd and even lines, and thus reduce the flickering effect. However, a problem with such flicker filters is that they also significantly decrease the vertical resolution of an image, causing blurring.